The present invention relates to a method for processing silicon devices and, in particular, to a method which uses in a hybrid optical thermometer system for measuring and controlling the temperature of a silicon workpiece as it is processed.
An apparatus for carefully controlling the temperature of silicon workpieces is an important part of silicon device fabrication. There have been many systems developed for use in fabricating silicon devices that essentially are designed to control the temperature of a workpiece while the processing is being carried out. A typical silicon workpiece heating station comprises a workpiece support, apparatus for measuring the temperature of the workpiece, and controllable heating elements, such as infrared bulbs. Electronic circuitry may receive the measured temperature and control the heating elements to achieve the desired temperature as a function of time.
With advances in silicon processing technology, there has been a corresponding need for improved workpiece heating stations and temperature control. One major difficulty relates to efforts to measure and control the temperature of a workpiece where temperatures below 500xc2x0 C. are used in the processing steps. Another difficulty relates to measuring and controlling the temperature of the workpiece that has been or is being modified with deposited materials. In fabricating integrated circuit devices, there are numerous ways in which a silicon workpiece may be processed that involve depositing materials onto the silicon workpiece or removing portions of these deposited materials. The presence of the fabrication materials used in such processing impacts upon the measurement of the workpiece temperature. Typically, in these methods the processing is performed on at least one surface of the workpiece, making it difficult to obtain accurate measurements of the workpiece as materials have been or are being applied thereon.
One approach for measuring and controlling the temperature of workpieces involves use of thermocouples. Although thermocouples are effective over a wide temperature range, to be accurate they must be attached directly to the workpiece; this attachment is costly, time-consuming, and presents risks of contaminating the workpiece. Optical pyrometers are advantageous for use in measuring and controlling the temperature because they do not have to be attached to the workpiece. However, they have reduced accuracy at lower temperatures; at temperatures below 500xc2x0 C., they lack the accuracy needed for silicon processing. Pyrometers also require a different calibration to convert their signal to a temperature depending on the silicon surface condition. Recently, improved pyrometers have been developed which invoke a ripple technique, taking advantage of the thermal modulation of the AC current which powers the heating lamps. Such improved pyrometers can be controlled to an accuracy of 12xc2x0 C. at temperatures near 1100xc2x0 C., and pyrometers using this technique are effective to near 600xc2x0 C., though with decreasing accuracy. But lower temperatures and higher accuracy are still needed, as is a method of accounting for the surface condition.
Thus, improved temperature measurement devices and methods are desired for use in silicon device fabrication which achieve greater accuracy at lower temperatures and may be used where materials have been or are being applied to the silicon workpieces.
Summarily described, the invention embraces a method for processing integrated circuits on silicon wafers, using a method for measuring the temperature of a silicon workpiece. A thermometry method that allows the control of integrated circuit processing comprises the steps of providing a conversion system for transforming spectral data to a value of temperature, and measuring this spectral data with optical reflectance. The optical reflectance thermometry comprises directing a beam of polarized light including ultraviolet light onto the silicon workpiece so the light is reflected off the workpiece to a spectrum analyzer; analyzing the spectrum of the light reflected off the workpiece to obtain spectral data; and converting the spectral data to information about the surface of the wafer. Using this information, a thermometry method is chosen, the temperature is determined, and the processing is controlled.
Using the inventive optical reflectance system, a hybrid thermometer system is provided to measure and control the temperature of a silicon workpiece while accounting for the presence of fabrication materials that have been or are being applied to the workpiece. This hybrid system involves use of a plurality of and preferably three types of thermometers, e.g., a thermocouple, a pyrometer, and a reflectance thermometer, disposed adjacent the silicon workpiece. The thermometers are utilized to obtain real-time spectral data accounting for the presence of fabrication materials. A library of spectral data (e.g., a xe2x80x9cspectrum libraryxe2x80x9d), is measured prior to the real-time application using known, controlled conditions. The real-time spectral data is compared to values in the spectrum library to determine the type and concentration of fabrication materials applied or being applied (e.g., the xe2x80x9csurface conditionsxe2x80x9d). A decision is then made based on the surface conditions as to how the temperature should be measured, e.g., with optical reflectance, a pyrometer, or a thermocouple, and the temperature is measured using the appropriately selected technique. The spectrum library may be created or enhanced by utilizing the steps of providing a test substrate and a plurality of test fabrication materials, measuring the temperature of the test substrate and test fabrication materials as the fabrication materials are applied using a thermocouple that is attached to the test substrate and optical reflectance thermometry, and decomposing the spectral data into a basis set of functions and coefficients. The values obtained in measuring known test fabrication materials may then be used to provide the spectrum library.